There has been recognition in the prior art that it is desirable to convert methane into a higher molecular weight hydrocarbon. For instance, it is known that it is normally commercially unfeasible to transport methane produced with crude oil or natural gas from a well site to a distant location for consumption as fuel. Often the transportation problems relate to the extremely low temperatures needed to liquefy methane or to liquefy a gas mixture containing large amounts of methane.
The composition of natural gas at the wellhead varies, but the major hydrocarbon present is methane. For example, the methane content of natural gas may vary within the range of from about 40 to 95 volume percent. Other constituents of natural gas may include ethane, propane, butanes, pentanes (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
It is normally very costly to separate other light hydrocarbons such as ethane from methane and it is undesirable to mix methane with other hydrocarbons prior to transport. The result has been that large amounts of methane are essentially disposed of in a wasteful manner as by flaring without utilization of the hydrocarbonaceous nature of the methane.
Thus there has been interest in developing processes for the conversion of gaseous fuels, including natural gas and methane, to easily transportable, less volatile, value-added products, methanol and syncrude. Syncrude is synthetic crude produced by first converting methane to a mixture of carbon monoxide and hydrogen, termed synthesis gas, followed by conversion of the synthesis gas via the Fischer-Tropsch reaction to paraffinic hydrocarbons.
A. H. Singleton, et al. in an article entitled “Conversion of Associated Natural Gas to Liquid Hydrocarbons”, in the Proceedings of the Natural Gas Conference, Emerging Technologies for the Natural Gas Industry, 1997, available for download at http://www.doe.gov/publications/proceedings/97/97 ng/ng97_pdf/NGP15.PDF on or before Jan. 16, 2002, describes a plant and process for converting associated gas to syncrude on a FPSO (Floating Production, Storage, Off-loading vessel) off-shore and mixing the syncrude with crude oil separated from the associated gas. The process includes steam reforming of methane to oxidatively produce synthesis gas followed by Fischer-Tropsch catalytic reaction of the synthesis gas to produce paraffinic hydrocarbons. This process has the disadvantage of requiring intermediate production of synthesis gas. Thus, it is an indirect process.
U.S. Pat. No. 6,005,011, issued to G. Henningsen, describes a plant and process for converting associated gas from crude oil to methanol at or near the wellhead. The process uses partial oxidation of the associated gas. Thus, the process has the disadvantage of requiring a source of oxygen.
U.S. Pat. No. 5,635,541, issued to A. R. Smith et al., and U.S. Pat. No. 6,117,916, issued to R. J. Allam, et al. each describe air separation units for generation of oxygen for partial oxidation of remote natural gas. These patents teach that operation of an air separation unit tend to require power to supply the thermodynamic work of air compression. Thus, use of air separation units has the disadvantage of tending to make demands of power.
Notwithstanding the above teachings, there remains a need for an economical non-oxidative direct process for conversion of natural gas to transportable liquids at the wellhead.